This invention relates to a surface acoustic wave device including surface acoustic wave elements connected in parallel.
As is known, generally, a low loss and a steep out-of-band cutoff characteristic are required in a surface acoustic wave filter of a surface acoustic wave device used for mobile communication.
For example, a multistage cascade-connected resonant filter is used as a surface acoustic wave element for a system that requires a steep filter characteristic since a low loss is considered to be important and adjacent channels are close to each other though its frequency band is wide, like a GSM (Global System for Mobile Communication)—IF (Intermediate Frequency) filter.
In the multistage cascade-connected resonant filter, however, the number of stages needs to increase to secure steepness and thus a loss becomes large. In order to obtain a broad frequency band, tuning is needed between the stages of the respective resonant filters. The resonant filter is therefore difficult to mount.
Another structure of the surface acoustic wave filter is disclosed in, for example, Jpn. Pat. Appln. KOKAI Publications Nos. 62-43204 and 9-214284. In the surface acoustic wave filters of these publications, the frequency band can be broadened by frequency intervals of a multiplex mode of a resonator in each stage.
However, there is a limit to widen the passband of the above filters since multiple resonance frequency intervals chiefly depend upon the electromechanical coupling coefficient proper to a piezoelectric substrate. Unless the phase of an unnecessary higher-order mode outside the frequency band of the mode is set opposite to that of a resonant filter on the other party, the unnecessary higher-order mode cannot be suppressed. For this reason, the in-band characteristic cannot be designed virtually under the control of the out-of-band characteristic. The filters are therefore limited to their use.
In order to provide directivity with the propagation characteristic of a surface acoustic wave, a surface acoustic wave filter using a surface acoustic wave element R (Resonant) SPUDT (Single Phase Uni-Directional Transducer) has recently been used widely since it is high in flexibility of design, low in loss, and easy to miniaturize. In the surface acoustic wave element RSPUDT, SPUDTs of comb electrodes formed in a unidirectional electrode structure are arranged such that the main propagation directions of excitation waves are opposed to each other.
In this type of surface acoustic wave filter, however, the internal reflection of an electrode finger is positively used in order to achieve a unidirectional property. Both the bandwidth and skirt characteristic of the filter depend upon the ratio of a forward unidirectional electrode to a backward unidirectional electrode, the reflection factors thereof, and the number of electrode fingers. Therefore, there is a limit to designing the bandwidth and the skirt characteristic independently of each other. In particular, it is difficult to make a broad bandwidth and a steep skirt characteristic compatible with each other.
The present invention has been developed in consideration of the above situation and its object is to provide a surface acoustic wave device that requires a low loss, a broadband, and a steep skirt characteristic.